1. Field of the Invention
The present invention relates to an improvement in surface treatment of a polyimide film and to a polyimide film having a thin metal layer. More particularly, it relates to a method of surface treating a polyimide film to provide a thin metal layer-laminated polyimide film which exhibits improved peel strength and, even after heated or humidified, retains relatively high peel strength and to a polyimide film having a thin metal layer.
2. Description of the Related Art
Aromatic polyimide film is widely used in electronic equipment such as cameras, computers, and liquid crystal displays.
In applying an aromatic polyimide film as a substrate material of flexible printed wiring boards (FPCs), tape automated bonding (TAB) tape carriers, etc., copper foil is bonded to the aromatic polyimide film via an adhesive such as an epoxy resin adhesive.
Although the aromatic polyimide film is excellent in heat resistance, mechanical strength and electrical characteristics, the adhesive is inferior in heat resistance and the like. It follows that the copper/polyimide laminate fails to take the full advantage of the characteristics possessed by the polyimide.
To overcome this problem, adhesiveless all-polyimide copper clad laminates have been developed. All-polyimide copper clad laminates are prepared by (a) depositing such metal as nickel or chromium on polyimide by vapor deposition and then electroplating with copper, (b) applying a polyamic acid solution to copper foil, followed by drying and imidation, or (c) heat press bonding polyimide film and copper foil via thermoplastic polyimide. However, it has been pointed out that these all-polyimide copper clad laminates have small adhesion strength or impaired electrical characteristics.
A polyimide laminate having a polyimide adhesive between polyimide film and a copper layer is known (U.S. Pat. No. 4,543,295). This technique is unapplicable to a biphenyltetracarboxylic acid-based polyimide having a low coefficient of linear thermal expansion because of small adhesive strength achieved.
Therefore, it has been proposed to use in roll lamination a thermally fusible polyimide obtained by using a specific aromatic diamine. Nevertheless, the resulting polyimide copper clad laminates still tend to have small peel strength after allowed to stand in a humidifying condition. It has been sometimes impossible with these methods to reduce the thickness of a metal layer, or there has been difficulty in freely changing the thickness of a metal layer.
JP-A-6-21157 proposes an improved plating method, in which a polyimide film is rendered hydrophilic by surface treatment with an aqueous solution of a permanganate, such as sodium permanganate or potassium permanganate, or a hypochlorite, such as sodium hypochlorite or potassium hypochlorite. According to the publication, the treated polyimide film is electroless plated with nickel or cobalt to form a thin metal layer, which is, if desired, further electroless plated with copper, and a copper layer is formed thereon by electroplating to produce a copper-clad polyimide substrate. It is claimed that, when the copper-clad polyimide substrate is left to stand in a high temperature environment for a long time, the reduction in adhesion strength is negligible.
The polyimide film used in JP-A-6-21157 is Kapton (available from Du Pont-Toray Co., Ltd.) based on pyromellitic acid component. The p resent inventors have revealed that the surface treatment of JP-A-6-21157 produces no effect when applied to polyimide film based on a biphenyltetracarboxylic acid component.
An object of the present invention is to provide a method of surface treating a polyimide film to impart improved metal adhesion so that a thin metal layer-laminated polyimide film prepared by using the treated polyimide film retains relatively high peel strength even after left to stand in a heating or humidifying condition.
Another object of the present invention is to provide a polyimide film having a thin metal layer which comprises a polyimide film having been surface treated by the method and a thin metal layer formed on the treated side of the polyimide film.
The present invention relates to a method of surface treating a polyimide film having a biphenyltetracarboxylic acid component which comprises treating the surface of the film by contact to a solution, preferably immersion in a solution, containing at least one of potassium permanganate and sodium permanganate and at least one of potassium hydroxide and sodium hydroxide and treating the film surface with an acid thereby to improve the adhesion to metal.
The present invention also relates to a polyimide film having a thin metal layer which is composed of a polyimide film having been surface treated by the above-described method to have improved adhesion to metal and a metal layer formed on the surface-treated side of the polyimide film by vapor deposition or a combination of vapor deposition and plating.
It is important in the present invention that surface treatment of a polyimide film having a biphenyltetracarboxylic acid component with a solution containing potassium permanganate and/or sodium permanganate and potassium hydroxide and/or sodium hydroxide by immersion, spraying or a like application method be combined with acid treatment. A laminate obtained by forming a thin metal layer on the polyimide film having a biphenyltetracarboxylic acid component having been subjected to the combined treatments exhibits improved adhesion. Even after the laminate is allowed to stand in a heating or humidifying condition, reduction in peel strength is suppressed.